Method and apparatus for processing ferroelectric crystal elements



Jan. 15, 1957 J. P. REMEIKA 2,777,188 METHOD AND APPARATUS FORPROCESSING FERROELECTRIC CRYSTAL ELEMENTS Filed Dec. 21, 1954 rm. 2 f

2: VOLTAGE saunas /24 27 L dd/E,

FIG. 7. v P

A c f IN VENTOR J REME/KA B) A T TORNE V United States Patent 6 METHODAND APPARATUS FOR PROCESSING FERROELECTRIC CRYSTAL ELEMENTS Joseph P.Remeika, Berkeley Heights, N. 1., assignor to Bell TelephoneLaboratories, Incorporated, New York, N. Y., a corporation of New YorkApplication December 21, 1954, Serial No. 476,745

7 Claims. (Cl. 29-2535) This relates in general to the improvement ofpulse storage units comprising ferroelectric crystals, and morespecifically, single crystals of barium titanate.

Circuits used in certain types of digital computers, pulse storage, andswitching devices feature, as their basic storage elements, crystalshaving properties known as ferroelectric. Such elements exhibit arelationship between applied electrical polarizing field and theresultant polarization which is similar to the hysteresis loop exhibitedby magnetic materials.

It has been found that for optimum performance in the types of circuitsmentioned above, the basic ferroelectric storage element should exhibita hysteresis loop which is substantially rectangular, indicating a lowcoercive force, a high remanent polarization, and a high ratio betweenthe charge storing capacities at different points in the cycle. Thesecharacteristics are best attained in single crystals of barium titanate,the electrical domains of which are substantially aligned in a singledirection with respect to the c, or optic axis of the crystal.

In preparing barium titanate crystals for storage application of thetypes mentioned, it is attempted during the growth process and bysubsequent etching treatments to bring about a substantiallyunidirectional alignment of the electrical domains. Such a process hasbeen described in detail in my application Serial No. 344,373, filedMarch 24, 1953 and in my article entitled Growth of BaTiOa SingleCrystals for Use as Storage Elements, Journal of the American ChemicalSociety, vol. 76, pp. 940, 941, February, 1954. Although crystalsprepared in accordance with the aforesaid process have often been foundto exhibit substantially uniform alignment of the domains, theircharacteristics are at times not entirely predictable in this respect,often because of strains existing in the crystal structure prior totreatment. Moreover, even when the optimum alignment is achieved duringpreparation, the domains are subject to misalignment during furtherhandling and processing, such as is usual in dimensioning the crystalelements and applying electrodes.

Prior art treatments for the removal of misaligned surface domains bythe use of chemically active etchants and strong electrolytes not onlyexert deleterious effects upon the crystal surfaces, but also invariablyproduce decompositional by-products which must later be removed.Further, it has been found impractical to immerse the treated crystalsin the strong electrolytes of the prior art, inasmuch as this operationcauses short-circuiting around the edges of the crystal, causing highcurrent densities, with the possibility that the crystal will crack.

Accordingly, it is the principal object of the present invention toimprove the domain alignment of ferroelectric crystal elements tofacilitate pulse storage.

A more specific object of the invention is to improve the domainalignment in barium titanate crystal elements by a treatment whichproduces no deleterious effects on the treated elements.

4 Patented Jan. 15, 1957 A further object of the invention is to providea treatment, characterized as above, which is adaptable for massproduction techniques.

These and other objects, which will be apparent from a study of thespecifications and drawings hereinafter, are attained in accordance withthe present invention by placing a thin crystal wafer to be treated on aconducting metal plate and immersing the combination in distilled water.A field of between 2,000 and 3,000 volts per centimeter is appliedacross the thickness of the wafer, between the supporting metal plateand a conducting probe which is adjacent, but not in contact with, theupper surface of the wafer. In a preferred embodiment, to be describedin detail hereinafter, the conductivity of the distilled water is abouttwo micromhos per centimeter, the electrodes are platinum, and thefrequency of the applied field 60 cycles per second, although the latteris not critical; in fact, direct current may be used. The field isapplied for an interval of about two seconds, and then reduced to zero.

The technique of the present invention has been found to have thefollowing advantages over prior art methods of ferroelectric domainalignment. (1) The distilled water does not react chemically with thecrystal surface, and therefore does not cause deterioration thereof, ordecompositional by-products which tend to form as deposits on thecrystal surface. (2) Since, for the purposes of the presently disclosedtreatment, it is unnecessary to apply solid electrodes to the crystalsurfaces, the align ment of domains on the crystal surface remainsrelatively undisturbed. (3) There is no danger of the distilled waterbath short-circuiting the edges of the crystal and hence, no danger ofproducing high current densities which might cause the crystal to heatup and crack during treatment. (4) Because of the simple form of theapparatus, the brief interval of treatment, and the absence of permanentconnections with the treated element, the apparatus and techniques ofthe present invention are readily adaptable to mass production.

The present invention will be better understood from a study of thedetailed specification hereinafter and the attached drawings, in which:

Fig. l is a view through a polarizing microscope showing the complexdomain structure in a barium titanate crystal element prior to treatmentin accordance with the present invention;

Fig. 2 is a cross-sectional view of a distilled water cell adapted forthe treatment of ferroelectric crystal elements in accordance with theteachings of the present invention;

Fig. 3 is a view, similar to Fig. 1, of a barium titanate crystalelement after treatment according to the present invention;

Fig. 4 show a hysteresis loop for a typical crystal of barium titanate;

Fig. 5 shows an ideal hysteresis loop for storage applications, and

Figs. 6 and 7 show hysteresis loops typical of crystal elements beforeand after the treatment of the present invention, respectively.

Applicant has disclosed in copending application Serial No. 344,373,filed March 24, 1953, and in his publication, hereinbefore identified, atechnique for producing a ferroelectric crystal characterized by a lowdielectric loss, low coercive force, high remanence, and a substantiallyrectangular hysteresis loop, which is adapted for use as a storageelement in computer and switching applications. The crystals derivedfrom the specific growth technique described in the foregoingapplication and in my cited publication consist of a principal componentof barium titanate with small amounts of iron integrated into thecrystalline lattice structure.

Many of the crystals grown in the manner described in my aboveapplication, when viewed through a polarizing microscope between a pairof polaroid films rotated for extinction, are found to be made up of alarge number of bright and dark bands such as indicated, for example, inFig. l of the drawings accompanying the present application. This isbelieved to be caused by the fact that part of the electrical domainsare oriented with their optic axes at 90 degrees to the other domains.The bright bands represent a domains in which the optic axes areparallel to the major surfaces, causing these domains to exhibitbirefringence in the thickness direction. The dark bands representdomains in which the optic axes are perpendicular to the major surfaces,and hence, exhibit no birefringence in the thickness direction.

lnorder to provide a crystal having optimum storage characteristics, i.e., low coercive force, high remanence, and a substantially rectangularhysteresis loop, it is necessary that all of the domains be aligned sothat the direction of easy polarization; that is, the c perpendicular tothe major faces of the treated crystal. This is accomplished inaccordance with the present invention by applying a voltage in thethickness direction of a crystal element to be treated, while the latteris immersed in a bath of distilled water. The apparatus for performingthis operation is shown in Fig. 2 of the drawings. It comprises a glassvessel on the inside lower surface of which is disposed a platinum plate21 which acts as a conductor. For illustrative purposes, applicantsnovel treatment will be described as applied to square wafers two milsthick and seventy-five mils on a side, which, expressed in metric units,would be .05 millimeter thick and 1.9 millimeters on a side. Thesewafers are cut from single crystals of barium titanate prepared in themanner described in detail in my above-mentioned copending applicationSerial No. 344,373, and in my publication, and etched with phosphoricacid prior to the present treatment. The crystal wafers 22 to be treatedare placed on top of the plate 21 in the vessel 20, which is filled withdistilled water 23 to a level several millimeters above the uppersurface of the crystals 22. A short piece of platinum wire 24, attachedto a flexible lead 25, serves as a second conductor, disposed adjacentthe upper surface of the crystal 22 under treatment. The platinum wire24 terminates in a spherical probe 26 having a radius of about amillimeter, which is positioned about a millimeter above the center ofthe upper surface of crystal 22. The reason for using platinum for boththe probe 26 and the base plate 21 is that platinum is chemicallyinactive, and hence, does not contaminate the distilled water bath. Theprobe 26 and the base conductor 21 are connected by leads and 27 acrossa potential source 29 adapted to supply a potential of between say,2,000 and 3,000 volts per centimeter of the thickness of the treatedcrystal. For convenience, one may use a potential source having afrequency of 60 cycles per second. However, the frequency is notcritical, and even direct current can be used successfully. Althoughsome of the field energy is lost through the water, enough chargeappears on the surface of the crystal to effect orientation. Mosteffective operation was observed when the conductivity of the distilledwater 23 was not greater than 2.1 micromhos per centimeter. The treatedcrystal 22, after being removed from the bath 23, was again observedunder the polarizing microscope in the same position as prior totreatment, and between polaroid films set for extinction. It was foundthat a crystal wafer, subsequent to treatment for an interval of a fewseconds, presents an almost entirely dark appearance, as indicated inFig. 3 of the drawing, the a hands being no longer observable. Thisindicates that the electrical domains are in nearly perfect alignmentthroughout the entire wafer.

Referring now to Fig. 4 of the drawings, a hysteresis loop is shownwhich is typical of that produced by 60 or optic axis, is

til)

cycles per second alternating voltage applied across an untreated bariumtitanate crystal. The electrical field strength E applied through thethickness of the crystal is plotted against the consequent polarization.

Starting from zero field and polarization at point 0, the curve rises tothe right, at first gradually, then rapidly, and finally slopesasymptotically to saturation at C. Removal of the positive field nowallows the polarization to fall to a positive value at A, which iscalled the remanent polarization. To reduce the latter to zero, anegative field must be applied, which is known as the coercive force,the magnitude of which depends on the pretreatment of the crystal. Byanalogy to the hysteresis loop of ferromagnetic materials, the remainderof the complete loop CADBC is obtained.

Referring now to Fig. 5, an ideal hysteresis loop is shown for amaterial suitable for switching or computer applications such as, forexample, memory cells of the type described in J. R. Andersonapplication Serial No. 254,245, filed November 1, 1951, now Patent2,717,372, issued September 6, 1955. It should be noted that thefundamental requirement for such a material is that it be subject toelectrical saturation by voltage pulses :L2E1 volts; but when theferroelectric material is in either state A or C, as indicated in Fig.5, the application of voltage pulses of :El volts will not besufiiciently high to change its final state. The capacitance of a singleferroelectric memory cell, which is represented by the slope of thehysteresis curve at any point, should thus always remain at a low valueC1 when positive or negative voltage pulses no higher than E1 areapplied. However, when positive or negative voltage pulses as high as2E1 volts are applied in a direction to reverse internal polarization,the state of the ferroelectric material should pass rather abruptly froma low capacitance region C1 to a high capacitance C2 and then on to alow capacitance C1 near saturation.

Assuming a cycle voltage were impressed across the crystal shown in Fig.l, which was in accordance with the teachings of my copendingapplication Serial No. 344,373 and of my cited publication and etched inphosphoric acid, but prior to orienting treatment in accordance with thepresent invention, the resultant hysteresis loop appearing on theoscilloscope screen would assume the form shown in Fig. 6 of thedrawings, in which polarization is plotted against applied voltage.After applicants distilled water orienting treatment, the crystal ischaracterized by a rectangular hysteresis curve of the type shown inFig. 7, which is the result of the nearly complete elimination of adomains. .lt is apparent from the showing that in the typical case, thevoltage required for saturation is markedly decreased, as is also thecoercive force, whereas the remanent polarization is significantlyincreased.

The following table shows ranges of parameter values for typicalcrystals tested.

SIgIfgLE CRYSTALS OF BARIUM 'lIlANATE AFTER OWTH IN POTASSIUM FLUORIDEFLUX AND ETOH- ING WITH H3PO4 It is to be understood that the figuresgiven above are indicative of the efiectiveness of applicants treatmentin typical cases, the degree of improvement depending to a large extenton the initial condition of the treated crystals, such as, for example,the presence or absence of strains and impurities in the crystallattice.

Although some of the foregoing values may be approximated by the methodsset forth in my earlier filed copending application Serial No. 344,373,the presently disclosed technique has certain advantages in addition tothose discussed earlier in the specification. An important one of theseis that the resultant domain alignment is substantially uniform over theentire treated element, as is apparent from Fig. 3. This contrasts withthe condition of the crystal element after prior art treatment in whichonly certain portions are in alignment, thus producing a markedvariation in the hysteresis characteristics as the electrodes are movedfrom one point to another over the surface.

Although a specific apparatus has been disclosed by way of illustratingthe principles of the present invention, it will be apparent to thoseskilled in the art that other forms may serve suitably for the practiceof the present invention. Moreover, although the present procedures werespecifically described with reference to treatment of barium titanatecrystals grown by the method set forth in applicants application SerialNo. 344,373 mentioned above, it will be apparent that the method andapparatus are applicable also to the processing of other ferroelectriccrystal elements prepared by any of the various known techniques.

What is claimed is:

1. An apparatus for uniformly orienting the domains in ferroelectricsingle crystal elements which comprises in combination, a bath ofdistilled water, and means including a pair of electrodes, which arechemically inert with respect to distilled water, disposed in said bathto direct an electrical field in the thickness direction of a crystalelement under treatment of an intensity to align a major portion of thedomains of said crystal in said thickness direction.

2. An apparatus for uniformly orienting the domains in ferroelectricsingle crystal elements which comprises in combination, a bath ofdistilled water having a conductivity not greater than approximately twomicromhos per centimeter, and means including a pair of electrodes,which are chemically inert with respect to distilled water, disposed insaid bath to apply an electric field of between 2,000 and 3,000 voltsper centimeter across the thickness of a crystal element undertreatment.

3. An apparatus in accordance with claim 2 in which the electrodes ofsaid pair comprise a flat plate for supporting the crystal element undertreatment and a spherical probe spaced apart slightly from the uppersurface of said crystal element.

4. An apparatus in accordance with claim 3 in which said electrodes areplatinum.

5. A method for uniformly aligning the domains in a ferroelectriccrystal element which comprises immersing said element in a distilledwater bath and while said element is immersed in said bath directing afield in a thickness direction in said element which is of an intensityto align a major portion of the ferroelectric do mains of said crystalin said thickness direction.

6. A method for uniformly aligning the domains in a ferroelectriccrystal element which comprises immersing said element in a distilledwater bath, and While said element is immersed in said bath directing anelectrical field of between 2,000 and 3,000 volts per centimeter in athickness direction in said element.

7. An apparatus for uniformly orienting the domains in ferroelectricsingle crystal elements which comprises in combination, a bath ofdistilled water having a conductivity not greater than approximately twomicromhos per centimeter, a pair of chemically inert electrodes disposedin said bath comprising a plate adapted to support one of said elementsunder treatment, and a conducting probe located centrally with respectto the upper surface of said one element and slightly spaced apart fromsaid surface, and a potential source connected across said electrodesand adapted to impress between 2,000 and 3,000 volts per centimeteracross the thickness of the treated crystal element.

References Cited in the file of this patent UNITED STATES PATENTS

